1. Field of the Invention
The present invention relates to a sheet conveying apparatus for conveying sheets and an image reading apparatus that is provided with the sheet conveying apparatus and reads images on the sheets conveyed by the apparatus.
2. Description of the Related Art
Conventional image reading apparatuses provided in digital copying machines, printers, and facsimiles include an image reading apparatus in which an Auto Document Feeder (ADF), which is one example of the sheet conveying apparatuses, is provided in such a way as to be freely opened or closed with respect to a platen glass and a original base glass in order to feed a document, which is one example of the sheets, automatically to the image reading apparatus.
Among the reading methods of the image reading apparatus provided with this ADF is the following reading method (hereinafter referred to as “flow reading”): reading means provided below the ADF is moved to a set position with respect to a document automatically fed by the ADF by a motor and is stopped there; then, light is emitted from a lamp unit to the document conveyed at a predetermined speed to scan the document; and then its reflected light is detected by reading means to read the image.
Further, in the case of a two-sided document is adopted the following three-circulation two-side reading method: a document is fed to a reading part where an obverse image is read by flow reading; then, the document is switched back by a sheet discharging part and is fed again to the reading part where a reverse image is read; and then, the document is again switched back by the sheet discharging part and then is again circulated to the reading part to change the direction of the document and then is discharged.
In order to improve productivity in reading the two-sided document, there is provided a high-speed two-side reading method for conveying a following two-sided document by utilizing time that elapses while a preceding two-sided document is switched back, that is, a conveyance path space. The method is disclosed, for example, in Japanese Patent Application Laid-Open No. 2000-143104. To be more specific, according to this method, an image on the reverse side of the Nth document is read and then the (N+1)th document is fed to the reading part where an image on the obverse side of the (N+1)th document is read; and then, the (N+1)th document is switched back while the Nth document of the third circulation is passed through the reading part.
However, in the case of performing the high-speed two-side reading, a path length needs to be elongated so as to prevent two documents from colliding with each other to elongate the time required to pass the path, which does not very much improve productivity. Further, the elongated path length presents a problem of increasing the size of the apparatus.
Further, if the construction shown in FIG. 12 is adopted to convey sheets in such a way that a preceding sheet and a following sheet pass each other when the preceding document is switched back, the path length can be shortened and an apparatus can be prevented from being increased in size, but the following defects remain.
In the state shown in FIG. 12, an image on the reverse side of the Nth document is read and the Nth document is switched back by a sheet discharging and reversing part X and is being conveyed to the second registration rollers 27′, 28′.
Further, the next (N+1)th document is being conveyed to the sheet discharging and reversing part X while an image on the obverse side is being read. At this time, the (N+1)th document travels while lifting a flapper 31′ by its front end.
Here, the Nth document passing the top surface side of the flapper 31′ is sandwiched between an upper guide and the flapper 31′ to become wavy at the sheet discharging and reversing part X to impair the entry of the (N+1)th document.
To prevent the document from becoming wavy, it is required to elongate an interval between the Nth document and the (N+1)th document in such a way that after the front end of the Nth document enters the second registration rollers 27′, 28′ and the sheet discharging and reversing roller is separated from the Nth document, the (N+1)th document enters the flapper 31′. However, this presents a problem of elongating a conveyance-path and increasing the size of an apparatus.
Further, if the Nth document and the (N+1)th document are conveyed in such a way that they pass each other, the Nth document and the (N+1)th document are put into sliding contact with each other. The sliding contact between the Nth document and the (N+1)th document applies load to the conveyance of the Nth document. Hence, to the Nth document are applied the load caused by the sliding contact with the (N+1)th document and the load caused by the pressure of the flapper 31′. There is a possibility that the load applied to the Nth document will impair also the conveyance of the Nth document.
In order to avoid the swing of the flapper from applying load to the document, a construction for preventing the flapper from swinging is disclosed in Japanese Patent Application Laid-Open No. 2003-95542. In the construction shown in FIG. 13, which is disclosed in Japanese Patent Application Laid-Open No. 2003-95542, switching members 460, 470 that construct a flapper is applied a force in the counterclockwise direction by a spring 471. A document is conveyed to the left side in the drawing by a conveyer roller 431 and the top surface of the document is guided by the bottom surface of the flapper 460.
When the document is guided and conveyed by the switching members 460, 470, the switching member 470 that is integral with the switching member 460 is swung by the force applied by the front end of the document. The swinging of the switching member 470 is limited by the lower protrusion 470a2 of a stopper 470a abutting against the lower wall surface 430b of a frame. The pressing force that the front end of the document applies to the switching member 460 opens a path on the downstream side of the swinging pivot 453 of the switching members 460, 470 and closes a path on the upstream side of the swinging pivot 453 to a position where a path to the downstream side is secured. That is, the construction disclosed in Japanese Patent Application Laid-Open No. 2003-95542 solves a problem that when the document moves the switching member, the conveyance of the document is impaired by the flapper.
However, as described by use of FIG. 12, there still remains the following problem: in the construction where Nth document and the (N+1)th document pass each other, the (N+1)th document swings the flapper; and the swung flapper impairs the conveyance of the Nth document.
In other words, the Nth document is pinched by the swung flapper 31′ to become wavy and the wavy Nth document prevents the entry of the (N+1)th document. Further, there are cases where the load caused by the Nth document being pressed by the flapper 31′ and the load caused by the sliding contact with the (N+1)th document are applied to the Nth document and this load applied to the Nth document is likely to impair the conveyance of the Nth document.